This is an open, randomized, multicenter, phase I/II study with a parallel group design investigating an intravenous and an oral CY dosing schedule in combination with LEN and low-dose DEX in patients with refractory or relapsed MM. In phase I MTD of CY will be determined using a common dose escalation scheme with 3 to 6 patients per dose level. In phase II, efficacy and safety of the treatment regimens at the MTD will be investigated in 20 patients.